The invention relates to an operational amplifier.
A transistor amplifier which includes two emitter-coupled NPN transistor pairs is known from DE-OS 30 27 071. In the common emitter lead of each transistor pair there is connected a respective constant current source, i.e. a direct current source, whose current is independent of the voltage at the base electrodes of the associated transistor pair. The other terminal of the constant current source is connected to ground. The collector electrode of a first transistor of the first transistor pair is connected to the collector electrode of a first transistor of the second transistor pair. Analogously, the collector electrodes of the second transistors of the transistor pairs are connected to one another. The common collector terminals are connected to the positive supply voltage via a respective resistor and at the same time constitute the output terminals of the amplifier circuit. Furthermore, the base electrode of the first transistor of the first transistor pair is connected to the base electrode of the first transistor of the second transistor pair via a direct voltage source. Analogously, the base electrodes of the second transistors of the transistor pairs are connected to one another via a further direct voltage source. These two direct voltage sources, constituting a short-circuit for alternating current signals, supply the same voltage with such a polarity that the base of the first transistor of the second transistor pair is more positive than the base of the first transistor of the first transistor pair and that the base of the second transistor of the first transistor pair is more positive than the base of the second transistor of the second transistor pair. Moreover, the base electrodes of the first transistor of the first transistor pair and of the second transistor of the second transistor pair are connected to a low-impedance signal source. All transistors have the same characteristics. Particularly small non-linear distortions are achieved by selecting the direct current for the second transistor of the first transistor pair and for the first transistor of the second transistor pair so as to be from three to ten times the direct current for the other transistors.
DE-PS 33 23 277 discloses a current amplifier which includes a so-called voltage-current converter circuit which comprises a difference input and a difference output as well as two current sources which supply respective equal currents. The voltage-current converter circuit includes a resistor which is connected to the output terminals of the current sources. PNP transistors are connected, by way of their emitters, to the junctions of the output terminals of the current sources and the resistor; they convert the voltage supplied via two input terminals into a current. The bases of second NPN transistors are connected to said input terminals whereas the emitters of the relevant transistors are connected to the bases of the above-mentioned PNP transistors. The latter PNP transistors operate as emitter followers which drive the NPN transistors. The latter transistors supply collector output currents. Moreover, two further PNP transistors are connected to the input terminals by way of their bases; they constitute a first start circuit. The emitters of said further PNP transistors are connected to the junctions of the resistor and the emitter electrodes of the former PNP transistors whereas the collectors of the relevant transistors are connected to the collectors of the former PNP transistors. The further PNP transistors do not operate continuously during operation of the circuit, but are driven merely at the instant of switching on, for example when the voltage supply switch or mains switch is actuated.
Such a circuit aims to provide a current amplifier in which the effects of the base current of a common-emitter transistor pair in a multiplier circuit can be eliminated so as to enhance the linearity and the accuracy of the multiplication factor of the relevant circuit.
The article "1-V Operational Amplifier with Rail-to-Rail Input and Output Ranges" by Jeroen Fondrie in IEEE Journal of Solid-State Circuits, Vol. 24, book 6, December 1989, pp. 1551 to 1559, describes a bipolar operational amplifier whose input and output driving ranges closely approximate the potentials of the current supply terminals. The operational amplifier shown therein is suitable for supply voltages as low as one volt. It should notably be possible for the output voltage to approximate the potential of the supply voltage terminals to within 100 mV. This operational amplifier includes two mutually complementary input stages, the first of which includes an emitter-coupled transistor pair of the NPN type whereas the second stage includes a similarly coupled transistor pair of the PNP type. The collector terminals of the transistors of the two transistor pairs are connected to a summing circuit for forming an output current. Both emitter-coupled transistor pairs receive, via their base electrodes, the voltages from two push-pull input terminals, that is to say via level shift resistors between the push-pull input terminals and the base terminals of the transistors. Overall, four level shift resistors are provided, i.e. a respective resistor between the non-inverting push-pull input terminal and the base terminal of each first transistor of the emitter-coupled pairs and two further terminals between the inverting push-pull input terminal and the two remaining base terminals. The four level shift resistors are activated by four current sources. Because the push-pull input terminals can no longer reach the potential of the positive or the negative supply voltage terminal in the case of an activated level shift, the current sources are controlled in dependence on the common mode input voltage. The current sources then deliver their maximum current when the common mode input voltage is at the centre of their driving range, and the current of the current sources becomes zero when the common mode input voltage approximates the level of one of the two supply voltage terminals. A very complex circuit arrangement is required so as to achieve this effect.